diff --git a/tianshou/core/mcts/mcts_virtual_loss.py b/tianshou/core/mcts/mcts_virtual_loss.py
index 9335464..f27d8a3 100644
--- a/tianshou/core/mcts/mcts_virtual_loss.py
+++ b/tianshou/core/mcts/mcts_virtual_loss.py
@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 # vim:fenc=utf-8
 # $File: mcts_virtual_loss.py
-# $Date: Sat Dec 23 02:4850 2017 +0800
+# $Date: Sun Dec 24 16:4740 2017 +0800
 # Original file: mcts.py
 # $Author: renyong15 © <mails.tsinghua.edu.cn>
 #
@@ -22,7 +22,17 @@ from .utils import list2tuple, tuple2list
 
 
 class MCTSNodeVirtualLoss(object):
-    def __init__(self, parent, action, state, action_num, prior, inverse=False):
+    """
+        MCTS abstract class with virtual loss. Currently we only support UCT node.
+        Role of the Parameters can be found in Readme.md.
+    """
+    def __init__(self, 
+                 parent, 
+                 action, 
+                 state, 
+                 action_num, 
+                 prior, 
+                 inverse = False):
         self.parent = parent
         self.action = action
         self.children = {}
@@ -41,7 +51,19 @@ class MCTSNodeVirtualLoss(object):
         pass
 
 class UCTNodeVirtualLoss(MCTSNodeVirtualLoss):
-    def __init__(self, parent, action, state, action_num, prior, inverse=False, c_puct = 5):
+    """
+        UCT node (state node) with virtual loss.
+        Role of the Parameters can be found in Readme.md.
+        :param c_puct balance between exploration and exploition,
+    """
+    def __init__(self, 
+                 parent, 
+                 action, 
+                 state, 
+                 action_num, 
+                 prior, 
+                 inverse=False, 
+                 c_puct = 5):
         super(UCTNodeVirtualLoss, self).__init__(parent, action, state, action_num, prior, inverse)
         self.Q = np.zeros([action_num])
         self.W = np.zeros([action_num])
@@ -53,7 +75,8 @@ class UCTNodeVirtualLoss(MCTSNodeVirtualLoss):
 
         self.mask = None
 
-    def selection(self, simulator):
+    def selection(self, 
+                  simulator):
         self.valid_mask(simulator)
         self.Q = np.zeros([self.action_num])
         N_not_zero = (self.N + self.virtual_loss) > 0
@@ -108,6 +131,9 @@ class UCTNodeVirtualLoss(MCTSNodeVirtualLoss):
 
 
 class ActionNodeVirtualLoss(object):
+    """
+        Action node with virtual loss.
+    """
     def __init__(self, parent, action):
         self.parent = parent
         self.action = action
@@ -156,6 +182,9 @@ class ActionNodeVirtualLoss(object):
 
 
 class MCTSVirtualLoss(object):
+    """
+        MCTS class with virtual loss 
+    """
     def __init__(self, simulator, evaluator, root, action_num, batch_size = 1, method = "UCT", inverse = False):
         self.simulator = simulator
         self.evaluator = evaluator
@@ -196,13 +225,19 @@ class MCTSVirtualLoss(object):
         self.bp_time = []
         while (max_step is not None and self.step < self.max_step or max_step is None) \
                 and (max_time is not None and time.time() - self.start_time < self.max_time or max_time is None):
-            self.expand()
+            self._expand()
             if max_step is not None:
                 self.step += 1
 
-    def expand(self):
+    def _expand(self):
         """
             Core logic method for MCTS tree to expand nodes.
+            Steps to expand node:
+            1. Select final action node with virtual loss and collect them in to a minibatch.
+               (i.e. root->action->state->action...->action)
+            2. Remove the virtual loss
+            3. Evaluate the whole minibatch using evaluator 
+            4. Expand new nodes and perform back propogation.
         """
         ## minibatch with virtual loss
         nodes = []